Jet engine noise suppressor



July 29, 1969 R. s. BENHAM JET ENGINE NOISE SUPPRESSOR 2 Sheets-Sheet 1Filed Dec. 28, 1966 INVENTOR. ROBERT B. BENHAM ATTORNEY July 29, 1969 R.B. BENHAM 3,458,008

JET ENGINE NOISE SUPPRESSOR Filed Dec. 28. 1966 2 Sheets-Sheet 2 29 fWZZ/ZZ A/ZZ/J/L/UAZ/UUUMLZaMUJ J l I INVENTOR ROBERT B. BENHAM BY mumATTORNEY United States Patent US. Cl. 181--43 3 Claims ABSTRACT OF THEDISCLOSURE The present disclosure relates to novel and improvedapparatus for suppressing noise created by a high velocity stream of gasthat discharges from a nozzle into the ambient atmosphere. The improvedapparatus takes the form of a diffuser that is longitudinally disposedopposite the exit end of the nozzle. The diffuser includes a convergentsection adjacent the nozzle and a plurality of successive silencingstages into each of which a plurality of chutes conduct air from thearea surrounding the exterior of the diffuser. Each diverging silencingstage and its associated intruding chutes are designed so as to maintaina substantially constant area for gaseous flow through the stage andthereby minimize the possibility of separation of flow from theperipheral boundaries of the stage and the generation of secondarynoise.

The present invention relates to novel and improved ap paratus forsuppressing noise created by a high velocity stream of gas thatdischarges into the ambient atmosphere. More specifically, it relates tonovel and improved noise suppressing apparatus for a jet engine with anafterburner during ground run-up testing operations.

Various types of devices have been used in the past to suppress thenoise generated by a jet engine during test run-up operations. Elbowdevices, for example, have been used where the flow is turned upwardly60 to 90 degrees to take advantage of the typical jet directivitypattern. Similarly, colander devices have been used to exploit thediffusion characteristics of screens placed across the jet stream. Suchmechanisms, however, while suitable at the 1300 F. temperatures ofmilitary power, have been found incapable of standing up under the 3000to 3500 F. temperatures of the afterburning jet. In order to adapt thesemechanisms to afterburner operation, it was found necessary to resort toelaborate water or air cooling apparatus. Moreover, these mechanismshave been found to pro duce relatively high levels of self-generatednoise.

It is therefore a principal object of the present invention to provide anovel and improved noise suppressing device which can be used withefficiency with an afterburning jet engine.

It is a further object of the invention to provide a novel and improvednoise suppressing device for an afterburning jet engine which requiresminimal use of supplemental cooling apparatus and which minimizesself-generated noise.

It is a further object of the invention to provide a novel and improvednoise suppressing device for a jet engine which need not be speciallystressed to accommodate large aerodynamic loads.

It is a further object of the invention to provide a novel and improvednoise suppressing device for a jet engine which produces low drag and noupsetting moment.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

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FIG. 1 is a perspective view of a preferred embodiment of the inventionwith a portion of the plenum chamber casing cut away;

FIG. 2. is a longitudinal cross-sectional view of the embodiment of theinvention shown in FIG. 1;

FIG. 3 is a cross-sectional view along line 33 of FIG. 2;

FIG. 4 is a cross-sectional view along line 4--4 of FIG. 2;

FIG. 5 is a cross-sectional view along line 5-5 of FIG. 2; and

FIG. 6 is an end view of the invention shown in FIGS. 1 and 2.

Referring now to the various figures of the drawing, it will be notedthat the improved noise suppressing device 3 of the invention isdisposed adjacent the discharge gas exitend of the nozzle 5 such thatthe nozzle 5 and the noise suppressor 3 have a common longitudinal axis.The noise suppressor includes a converging section 7 adjacent the nozzle5 and four successive silencing stages 9, 11, 13 and 15. The convergentsection 7 is contoured and spaced from the end of the nozzle 5 so as tolower the static pressure of the exhaust gases from nozzle 5 and drawair from the surrounding atmosphere to sulficiently cool the interiorwalls of the section. The first silencing stage 9 of the diffuser 3 isconnected to the downstream end of section 7 in any suitable manner andprogressively increases in cross-sectional area from its inlet end toits outlet end. Three channels or chutes 17, which are triangular incross-section, are formed at equally spaced positions about theperiphery of the cylindrical wall of stage 9 and project progressivelyfurther into the nozzle discharge gas channel such that theircross-sectional areas increase from small apexes at the inlet end ofstage 9 to cross-sectional areas of predetermined enlarged size at theiroutlet end. Stage 9 and its chutes 17 are designed such that the primaryfluid flow area through the stage remains constant throughout itslength. The bellmouth device 19 is secured to the outer peripheralsurface of the inwardly fluted cylindrical wall of stage 9 preferablyapproximately midway between its inlet and outlet extremities and, aswill be more apparent hereinafter, accelerates the secondary flow of airthrough the chutes of stage 9 into the interior of the diffuser.

Stage 11 of the diffuser 3 is connected to the downstream end of stage 9in any suitable manner and continues the increase in the cross-sectionalarea of the gas and air discharge channel initiated by the precedingstage 9. Six channels or chutes 21, which are also triangular incross-section, are formed at equally spaced positions about the innerperiphery of stage 11 and are uniformly staggered with respect to thechutes of stage 9 so as to more readily affect the flow between adjacentpairs of chutes 17 in stage 9. As in stage 9, the chutes 21 of stage 11project progressively further into the gas flow channel such that theircross-sectional areas increase from small apexes at the inlet end ofstage 11 to cross-sectional areas of predetermined enlarged size at theoutlet end. Stage 11 and its chutes 21 are designed such that theprimary fluid flow area through the stage remains constant throughoutits length. The bellmouth device 23 is secured to the outer peripheralsurface of the inwardly fluted cylindrical wall of stage 11 preferablyapproximately midway between its inlet and outlet extremities.

The third silencing stage 13 of the diffuser 3 is connected to theoutlet end of stage 11 in any suitable manner and continues the increasein the cross-sectional area of the gas and air discharge channelinitiated by the preceding stages. Twelve triangular channels or chutes25 are formed at equally spaced positions about the inner periphery ofstage 13 and are uniformly staggered with respect to the chutes of stage11. As in stages 9 and 11, the chutes of stage 13 project progressivelyfurther into the gas flow channel from small apexes at the inlet end ofstage 13 to chutes of predetermined enlarged size at the outlet end.Stage 13 and its chutes 25 are also designed such that the primary fiuidflow area through the stage remains constant throughout its length. Thebellmouth device 27 is secured to the outer peripheral surface of theinwardly fluted cylindrical wall of stage 13 preferably approximatelymidway between its inlet and outlet extremities.

The fourth silencing stage 15 of the diffuser 3 is connected to theoutlet end of stage 13 in any suitable manner and continues the increasein the cross-sectional area of the gas and air discharge channelinitiated by the preceding stages. Twelve triangular channels and chutes28 are formed at equally spaced positions about the inner periphery ofstage 15 and are uniformly staggered with respect to the chutes of stage13. As in the preceding stages, the chutes of stage 15 projectprogressively further into the gas flow channel from small apexes at theinlet end of stage 15 to chutes of predetermined enlarged size at theoutlet end. Stage 15 and its chutes 28 are also designed such that theprimary fluid flow area through the stage remains constant throughoutits length.

The outer casing 29 is positioned in any suitable manner about theexterior surface of the various stages of the diffuser and defines theplenum chamber 31 therebetween. The annular plenum chamber end plates 33and 35 are secured to and extend radially outwardly respectively fromsection 7 and stage 15 and are preferably spaced a predetermined smalldistance from opposite ends of the casing 29 so as to permitunrestricted flow of air between the plenum chamber and the externalatmosphere but limit any line of sight sound path between any bellmouthdevice and the external atmosphere. The casing 29 and the plenum chamberend plates 33 and 35 are lined with fiber glass or another suitablesound absorbent material. Although not shown in the drawing, the casing29 also includes labyrinth openings to further provide unrestricted airinlet but no line of sight sound path between the plenum chamber 31 andthe exterior.

In operation, the engine exhaust gases from nozzle enter the convergingsection 7 of the suppressor 3 and in doing so, draw cooling air from thesurrounding atmosphere along the inner walls of section 7. The size ofthe flow discharge passage of section 7 at its outlet end is designed soas to pass the afterburner gas flow plus just that amount of cooling airnecessary to adequately cool the inner walls of section 7. The staticpressure of the exhaust gases passing through section 7 is subtantiallyreduced from their ambient value at the junction of nozzle 5 and ection7 to promote the induction of mixing air from the plenum chamber 31through the various chutes of succeeding stages of the suppressor. Theexhaust gases and cooling air then progress into the first stage of thesuppressor. At the inlet end of stage 9, the chutes 17 begin to protrudeinto the gas and air flow. Although not shown in the drawing, the wallsof stage 9 and its chutes 17 are cooled by thin air films bled from theplenum chamber 31. The chutes open into the interior of the suppressorat the exit end of stage 9 and in so doing, direct air from the plenumchamber 31 into the primary gas and air stream where they mix and reducethe velocity of the primary stream. The small number of chutes of stage9 permit the introduction of only enough secondary air to reduce theprimary flow velocity a predetermined small amount where mixing shearsare high due to the high velocity of the primary flow. The bellmouthdevice 19 -accelerates secondary flow into the chutes and insuresmaximum momentum recovery. Design of the diverging stage 9 and itsprotruding chutes 17 so as to maintain a substantially constant area forthe primary gas and air flow through stage 9 minimizes the possibilityof separation of flow from the peripheral boundaries of stage 9 and thegeneration of secondary noise.

The primary gas and air stream then passes through the second stage ofthe suppressor. The chutes 21 begin to protrude into the primary streamat the inlet ends of stage 11 and open into the stream at their exitends. Air from the plenum chamber 31 directed into the primary stream bythe chutes 21 mixes with the primary stream and further reduces itsvelocity. The increased number of chutes of stage 11 introducesincreased amounts of air from the plenum chamber 31 and effects agreater reduction in the velocity of the primary stream. The bellmouthdevice 23 accelerates secondary flow into the chutes 21 and insuresmaximum momentum recovery. Design of the diverging stage and itsprotruding chutes 21 to maintain a substantially constant area for theprimary gas and air flow through stage 11 again minimizes thepossibility of separation of flow from the peripheral boundaries ofstage 11 and the generation of secondary noise.

The primary gas and air stream then passes through the third stage ofthe suppressor. Air from the plenum chamber 31 which is directed intothe primary stream by the chutes 25 that open into the stream at theexit end of the suppressor, mixes with the primary stream and stillfurther reduces its velocity. The bellmouth device 27 acceleratessecondary flow into the chutes 25 and insures maximum momentum recovery.The uniformly staggered relationship of the chutes of stages 9, 11 and13 assures thorough mixing of the primary and secondary streamsthroughout the entire radial field of flow and increases the efficiencyof the velocity reduction and noise silencing process. As the number andcross-sectional areas of the chutes increase and the mixing channelsnarrow in stages 9, 11 and 13, the mixing turbulence is damped out and afairly uniform velocity profile is produced when the flow is exitedthrough the ambient exit daisy nozzle of stage 15.

Ambient air is supplied to the plenum chamber 31 through the openingsbetween the ends of casing 29 and the plenum chamber end plates 33 and35 and through labyrinth openings in casing 29 not shown in the drawing.The inner walls of the casing 29 and end plates 33 and 35 are lined witha suitable material to absorb sound radiated from the various componentstages of the suppressor. End plates 33 and 35 block line-of-sight soundpaths between the bellmouth devices 19, 23 and 27 and the exterior.

Although the invention has been described hereinabove as havingspecified numbers of chutes in each stage and a specific number ofstages in the diffuser, it is to be understood that any greater orlesser number of chutes and/ or stages could be designed into thediffuser depending on desired noise suppression requirements withoutdeparting from the spirit or scope of the invention.

What is claimed is:

1. Apparatus for suppressing noise of a high velocity stream of gasissuing from a nozzle, said apparatus comprising:

(a) a diffuser which is positioned adjacent the exit end of the nozzlesuch that the nozzle and the diffuser have a common longitudinal axis,said diffuser including a convergent section adjacent the nozzle and atleast three successive silencing stages, each said silencing stagehaving a plurality of chutes that conduct air from the area surroundingthe exterior of the diffuser into the diffuser primary gas stream, eachof the said chutes being generally triangular in section across thelongitudinal axis of the diffuser and projecting progressively furtherinto the nozzle discharge gas stream as the gas moves downstream, theeffective cross-sectional area of the diffuser throughout the length ofeach of its first, second and third silencing stages being substantiallyequal to the cross-sectional entrance area of its respective stage;

(b) means for forming a plenum chamber about the a line of sight soundpath from either bellmouth strucexterior surface of the diffuser fromwhich the air ture and the exterior of the plenum chamber. directedthrough the chutes is supplied;

(c) and individual bellmouth structures for the chutes References Citedof the second and third stages that open into the 5 UNITED STATESPATENTS plenum chamber. 2. The apparatus substantially as described inclaim 1 2974744 3/1961 Wade wherein the means for forming a plenumchamber in- FOREIGN PATENTS eludes a circumferential cylindricalstructure, the inner 889,688 2/1962 Great Britain peripheral wall ofwhich is lined with a sound absorbent 10 matefial- ROBERT S. WARD, JR.,Primary Examiner 3. The apparatus substantially as described in claim 2wherein the means for forming the plenum chamber fur- US. Cl. X.R.

ther includes sound absorbent lined end walls that prevent 18151

